The present invention relates to a process for producing diaryl carbonate, and more particularly, to a process for producing diaryl carbonate, in which hydrogen chloride by-produced by phosgene method is converted into chlorine and then is used in the production of phosgene.
Large amounts of diaryl carbonates have been used as raw materials for producing aromatic polycarbonates by subjecting the diaryl carbonate to transesterification with bisphenol A. As the method for the production of diaryl carbonate, there is well known such a method comprising producing diaryl carbonate from dialkyl carbonate, and recovering and recycling by-produced alkyl alcohol. However, in this method, there are caused various inconveniences such as difficult separation of respective reaction products, discoloration of aromatic polycarbonates obtained from the diaryl carbonate due to impurities by-produced upon the transesterification reaction, production of undesired cross-linked aromatic polycarbonates or the like. Therefore, it has been required to improve these methods.
On the other hand, as methods of producing diaryl carbonate from phosgene, there are known many methods comprising using a homogeneous catalyst or a heterogeneous catalyst. However, in these phosgene methods, many unsolved problems still remain, for example, such that by-produced aryl salicylate has a boiling point which is extremely close to that of the aimed diaryl carbonate, a catalyst recovery step is required when using the homogeneous catalyst, or metal components is eluted when using the heterogeneous catalyst. Also, in a method of using an activated carbon catalyst, although this method is free from the problem as to elusion of metal components, it is difficult to achieve an industrially useful reaction rate.
It is also known to produce diaryl carbonate by reacting an aromatic monohydroxy compound with phosgene. For example, in U.S. Pat. No. 2,837,555, there has been proposed the method of conducting a solvent-free condensation reaction in the presence of halogenated tetramethyl ammonium as a catalyst. However, in this method, the use of a relatively large amount of the catalyst and a relatively high reaction temperature ranging from 180 to 215xc2x0 C. are required to achieve an economically useful reaction rate. Therefore, the method tends to cause the decomposition of thermally unstable halogenated tetramethyl ammonium. Besides, in this method, the phosgene is consumed in such a large amount far exceeding the stoichiometric amount thereof.
In Japanese Patent Publication (KOKOKU) No. 58-50977(1983), there has been proposed the method of producing diaryl carbonate (diphenyl carbonate) by reacting 2 moles of an aromatic monohydroxy compound (phenol) with one mole of phosgene at a temperature of 40 to 180xc2x0 C. using an aromatic nitrogen-containing heterocyclic compound as a catalyst optionally in a solvent, while eliminating hydrogen chloride from the reaction system. In this method, the reaction temperature is lower than that used in the above U.S. Patent, and the reaction rate is higher by two or more times than that described in the above U.S. Patent.
Thus, the method as described in Japanese Patent Publication (KOKOKU) No. 58-50977(1983) is industrially advantageous for the above-described reasons. However, in this method, the production rate of diphenyl carbonate upon the reaction between phenyl chloroformate as an intermediate product of the diphenyl carbonate and phenol, is low as compared to the production rate of the phenyl chloroformate. As a result, after completion of the reaction, a small amount of phenyl chloroformate still remains in the reaction system together with the diphenyl carbonate. The residual phenyl chloroformate adversely affects the quality of aromatic polycarbonate produced from the diaryl carbonate, as described above.
In order to industrially produce a high-quality polycarbonate, there has been studied a method of producing high-quality diaryl carbonate containing no aryl chloroformate. It is known that this object can be achieved by the following two-stage process. That is, in the process for producing diaryl carbonate comprising reacting the aromatic monohydroxy compound with phosgene in the presence of the aromatic nitrogen-containing heterocyclic compound, the phosgene is used in an amount of 0.44 to 0.5 equivalent based on one equivalent of the aromatic monohydroxy compound so that diary carbonate and aryl chloroformate are produced in such an equivalent amount less than that of unreacted aromatic monohydroxy compound; and then the aryl chloroformate and aromatic monohydroxy compound contained in a reaction solution obtained from the above step, are then subjected to dehydrochlorination reaction to produce diaryl carbonate (refer to Japanese Patent Application Laid-Open (KOKAI) No. 9-278714 (1997)). Further, in the above KOKAI, it is simply described that hydrochloric acid by-produced upon the above reaction is reused in plant, but the purpose of use of the by-product is not clearly specified therein.
On the contrary, the diaryl carbonate production process of the present invention, constitutes a closed cycle capable of reusing the by-product, and more specifically, it is to provide an ideal process in which the by-produced hydrogen chloride is reused for the production of phosgene.
Thus, in the process for producing diaryl carbonate by reacting the aromatic monohydroxy compound with phosgene using the aromatic nitrogen-containing heterocyclic compound as a catalyst, an industrially advantageous high-conversion ratio can be achieved; no salicylate-based impurities are produced unlike conventional methods of producing diaryl carbonate from dialkyl carbonate; and the aimed product can be readily separated from the by-produced monomers, so that it is possible to obtain diaryl carbonate having an extremely high purity at a high yield.
Hitherto, there have been conducted many studies as to methods for converting hydrogen chloride into chlorine. In particular, a large number of patent applications concerning oxidation reactions using a chromium oxide catalyst have been filed (refer to Japanese Patent Publication (KOKOKU) Nos. 6-15402(1994), 5-3402(1993), 5-3403(1993), 5-3405(1993), 5-3406(1993), 5-69042(1993) and 6-17203(1994)). Recently, many patent applications concerning oxidation reactions using a ruthenium oxide catalyst have also been filed (refer to Japanese Patent Application Laid-Open (KOKAI) Nos. 9-67103(1997), 10-194705(1998), 10-182104(1998), 10-338502(1998), 11-180701(1999) and 2000-34105).
However, in these prior arts, although the methods of converting hydrogen chloride into chlorine are described, there is neither taught nor suggested purposes of use of the converted chlorine, especially there are no prior documents teaching all steps of from the production of phosgene from the recovered chlorine to the production of diaryl carbonate.
An object of the present invention is to provide a process for producing diaryl carbonate by reacting an aromatic monohydroxy compound with phosgene using a catalyst comprising an aromatic nitrogen-containing heterocyclic compound, in which the diaryl carbonate can be produced at a high conversion ratio in an industrially advantageous manner, the by-produced hydrogen chloride is converted into chlorine, and the converted chlorine is used in phosgene production process.
As a result of the present inventors"" earnest studies concerning the purity of purified hydrogen chloride required for the reaction for converting hydrogen chloride into chlorine, the purity of recovered chlorine required for reusing it for the production of phosgene, the purity of the obtained phosgene required for the reaction with the aromatic monohydroxy compound, the concentration of impurities required for achieving the closed cycle process, and the position to which the impurities are purged off, the present invention has been attained based on the finding.
That is, in a first aspect of the present invention, there is provided a process for producing diaryl carbonate, comprising steps of:
(1) a diaryl carbonate production step of subjecting an aromatic monohydroxy compound and phosgene to condensation reaction at a temperature of 80 to 180xc2x0 C. in the presence of an aromatic nitrogen-containing heterocyclic compound catalyst to obtain the diaryl carbonate;
(2) a purification step of removing impurities from a hydrogen chloride gas by-produced upon the condensation reaction;
(3) a chlorine recovery step of recovering chlorine usable for the production of phosgene from the purified hydrogen chloride gas; and
(4) a phosgene production step of reacting the recovered chlorine with carbon monoxide to obtain phosgene usable in the diaryl carbonate production step.
In a second aspect of the present invention, there is provided the process for producing diaryl carbonate, having the following features.
(a) In the chlorine recovery step, the hydrogen chloride gas is subjected to oxidation reaction using oxygen at a temperature of 300 to 500xc2x0 C. in the presence of a chromium oxide catalyst under such a condition that the oxygen is used in an amount of not less than 0.25 mole based on one mole of the hydrogen chloride.
(b) After removing hydrogen chloride and water from a crude chlorine gas obtained by the oxidation reaction, the resultant chlorine gas is compressed and liquefied to obtain chlorine usable in the phosgene production step, and after recovering chlorine gas from unliquefied chlorine gas, the recovered chlorine gas is recycled to said compression and liquefaction step.
(c) A part of an oxygen-containing gas obtained after removing chlorine from the unliquefied gas, is discharged as an off-gas, and the remainder of the oxygen-containing gas is recycled to the oxidation reaction.
(d) In another chlorine recovery step, the purified hydrogen chloride gas is absorbed into water to prepare an aqueous hydrogen chloride solution having a predetermined concentration, and the aqueous hydrogen chloride solution is subjected to electrolysis.
(e) After removing water from a crude chlorine gas obtained by the electrolysis, the resultant chlorine gas is compressed and liquefied to obtain chlorine usable in the phosgene production step, whereupon a part of a circulating catholyte in an electrolytic cell is used for absorbing the purified hydrogen chloride gas thereinto.
(f) In other chlorine recovery step, the hydrogen chloride gas is subjected to oxidation reaction using oxygen at a temperature of 200 to 400xc2x0 C. in the presence of a ruthenium oxide catalyst under such a condition that the oxygen is used in an amount of not less than 0.25 mole based on one mole of the hydrogen chloride.
(g) After removing hydrogen chloride and water from a crude chlorine gas obtained by the above-mentioned oxidation reaction, the resultant chlorine gas is compressed and liquefied to obtain chlorine usable in the phosgene production step, and a part of the unliquefied oxygen-containing gas is discharged as an off-gas, and the remainder of the unliquefied oxygen-containing gas is recycled to the oxidation reaction.
(h) In the phosgene production step, a crude phosgene gas obtained by reacting chlorine with carbon monoxide is cooled and liquefied to obtain phosgene usable in the diaryl carbonate production step, and the unliquefied gas is discharged as an off-gas after completely decomposing phosgene.